%autoreload magics is not escaped

Some of my notebooks use
```
%load_ext autoreload
%autoreload 2
```

In the text representation of notebooks, the second line is not escaped, as `autoreload` is not a standard Jupyter magic.

List of magics to be escaped should be configurable, and include per default the most standard magics, so that I don't have to add `#escape` to the second line (current workaround).